For searching a salvage and probing a flying object by aircrafts, pilots perform searching (probing) of a rescue boat on the sea surface and the flying object by radio wave signals by a beacon such as a distress call, searching by visual inspection while flying over a searching sea area at a low altitude, searching based on an infrared camera image, etc.
In particular, the visibility and the viewing field become poor in nighttime and in a case of bad weather compared to searching done in a fine weather and in daytime. Thus, it is difficult to find the target object such as the salvage floating on the sea surface, the flying object, or the like by searching with the naked eyes and to capture it as an image detectable by an infrared camera.
This causes a delay in finding the search target, which may result in increasing the risk of allowing intrusions of suspicious characters such as navy ships and aircrafts of other countries.
For this, there have been developed an image information processing technique and an image identification support technique which effectively support viewing field searching by a prober through generating viewing field support information (image) for supporting a searcher to identify/discover the target object at an early stage by improving a contrast in the captured infrared image.
For example, known are: an infrared camera reduced in size through omitting a cooling function, which captures infrared images by a non-cooled infrared sensor in which the S/N ratio for a noise is improved largely; an image processing system using such infrared camera; and the like.
Further, as a related technique, there is disclosed an infrared image processing system which captures an infrared image by loading a small-sized non-cooled infrared camera on an aircraft or a flying object, processes the acquired infrared image, extracts/identifies the target object from the infrared image, and displays it (Patent Document 1).
Patent Document 1 separates infrared rays made incident from a target and from the background of the target into plan polarized light rays orthogonal to each other, receives the infrared rays by an image sensor in which two light receiving elements are arranged two-dimensionally, converts the infrared rays into electric signal amounts, and outputs a polarized light difference image signal acquired by finding a difference between electric signal amounts at a same coordinate position on two-dimensional coordinate between each of the image signals outputted after being converted into the electric signal amounts.
Further, a binary image signal acquired by binarizing the electric signal amount of the polarized light difference image signal based on a comparison with a threshold value is outputted by a combination of a polarized image capturing device which outputs a luminance image signal acquired by adding the electric signal amounts at the same coordinate position on the two-dimensional coordinate, and a threshold value database which stores a combination of the target and the background and the threshold value of the polarized light difference image signal with which the target and the background can be separated in a corresponding manner.
Further, a target candidate is extracted by an image that is acquired by multiplying the value after performing binarization on the binarized image signal to the electric signal amount of a luminance image signal at the same position on the two-dimensional coordinate.
The invention depicted in Patent Document 1: uses the fact that there is a temperature difference in a target object and the background thereof in general when extracting the target such as floating object floating on the sea surface, a ship, a flying object in an airspace, and an obstacle of a low visibility in a low airspace; includes an optical system constituted with a surface deflection light receiving element which separates into P-wave and S-wave loaded in front of an infrared ray sensor; includes an appropriate captured image luminance calculation processing unit; and performs infrared image processing which extracts a target object by defining a point that is equal to or more than a prescribed temperature (luminance) as the target object.    Patent Document 1: Japanese Unexamined Patent Publication 2007-322374
However, with the invention depicted in Patent Document 1 described above, it is difficult to reduce the size since the optical system for separating into the P-wave and S-wave before the infrared ray sensor is a large-scaled structure.
Further, the background noise in the infrared captured image captured by an infrared sensor becomes large in a case where the temperature difference between a target and the background thereof becomes small when the background is heated partially by the sun light or the like or in a case where the noise such as the sea spray on the water surface is extensive. Thus, the contrast between the target object and the background cannot be acquired depending on the optical system, so that it becomes difficult to recognize/capture the target correctly. This results in having such inconvenience that the target object is misrecognized or overlooked.
Further, in cases where there are white-crested waves generated due to the bad weather or artificial sea sprays and the like generated by hovering of a helicopter, the target object shown in an infrared captured image is covered by the white-crested waves. Thus, the shape of the target appears different. Therefore, when the target object identification is done by image identification based on the infrared captured image, it is possible to cause such inconvenience that the target object is overlooked or a wrong target object is selected (screening).
Furthermore, when different background regions such as a sea area (sea surface part) as the background of the target object, a fair part, a cloudy part, and the like in an airspace are contained in a captured infrared image, the contrast between the target object and the background cannot be improved according to each background region. Thus, the target object with respect to the background region becomes more indistinct, so that it is possible to cause such inconvenience that the target object is misrecognized or overlooked.
An object of the present invention is providing an image target identification device, an image target extraction method, and an image target extraction program for improving the inconveniences of the related technique and performing image processing so as to securely identify the target object within an infrared image.